Gellan gum is one of the best thickening agents or stabilizing agents in term of performances so far, and has good gelation property. The gel of gellan gum is easy to use, has a good flavor-releasing property, has a relatively high thermal stability, is easy to melt in mouth, has a high transparence, and has a controlled gelation time and gelation temperature. And, the gelation of gellan gum is not easy to be influenced by pH. The product is stable and has a lot of texture properties with variations. The gel also has other good characteristics.
Gellan gum is a hydrophilic colloid produced by the fermentation of a bacterium, Sphingomonas paucimobilis, which is a strain screened and separated from the nature by a broad selection. Gellan gum has a lot of useful properties.
The molecular basic structure of the gellan gum is a main chain, constituted by the repeating units of four sugars. The basic structure of a gellan gum molecule is a main chain, constituted by the repeating four sugars units. The single sugars participating a repeated unit include D-glucose, D-glucuronic acid, D-glucose, and L-rhamnose. In the natural form of this polysaccharide, each unit has 1.5 O-acyl groups, wherein each unit has 1 O-glyceryl substituent and every two units have 1 O-acetyl substituent. The glyceryl substituent is located on Position 2 of glucose residuals linked by a bond via 3 linkages, and the acetyl substituent is located on Position 6. These acyl groups are easily removed with a base, and thereby the natural form of gellan gum can be transferred into the product in the deacylated form. In contrast with the deacylated form, the natural form of gellan gum can also be called as high acyl product.
The high acyl gellan gum contains 11˜13% glyceryl groups and 4˜5% acetyl groups, with the total amount of the acyl groups being in the range of 15˜18% (weight percent); and, the low acyl gellan gum contains less than 1% of glyceryl groups and less than 1% of acetyl groups, with the total amount of the acyl groups being below 2%.
The acyl groups have a significant influence on the properties of the gel. The high acyl-type gellan gum can produce a gel which is soft, elastic, but does not have a brittleness while the low acyl-type gellan gum can produce a gel which is firm, does not have elasticity, but with a high brittleness.
At present, the low acyl gellan gum is broadly used in the fields, such as food industry and tissue culture media and so on. But, as to the process for preparing the low acyl gellan gum products, particularly as to the process for preparing the low acyl gellan gum products useful in these applications as mentioned above, there are only a few reports.
The low acyl, and non-clarified gellan gum product contains a part of proteins, with the amount of proteins being about 17% (weight percent). Therefore, it is not transparent. But, the low acyl and clarified gellan gum product has subjected to a filtration treatment, which removes a large part of the insoluble substances. So, the gel formed thereof is clear and transparent, having a super high transmission of light. Since the low acyl and clarified gellan gum is often used in the applications, such as food industry and so on, the tem “gellan gum”, in general, means the low acyl and clarified product in most circumstances.
At present, one defect of the existing processes for producing the low acyl and clarified gellan gum lies in that there is a difficulty in the complete removal of proteins from the colloid. Thereby, the transmittancy of the product is certainly influenced, and normally it is only above 80%. At the same time, the low acyl and clarified gellan gum as produced contains some divalent metal cationic ions, which makes the gellan gum product have difficulties in dissolving, and makes the gel as formed have a slightly white color, thereby influencing the quality of the product. At the same time, the traditional process for preparing the low acyl gellan gum products uses divalent or multivalent alkali metals salts to flocculate the gellan gum product in high acyl condition, but the high acyl gellan gum has a lower sensitivity to divalent or multivalent alkali metals salts, which is largely less than the low acyl gellan gum. So, the efficiency of flocculation is not high and the yield of product is influenced.
At present, the gelling agent for tissue culture medium usually is agar. But, the impurities and the hindering elements (including the sulfur element) in agar can influence the growth of plant tissue. So, the low acyl and clarified gellan gum product is a substitute for agar, having a great promise. Because the purity of gellan gum is very high and gellan gum is produced by industrial fermentation, the product has a more stable quality in view of the product obtained by extraction of agar from algae. The gelation time for gellan gum after it has been formulated into a solution is relatively shorter than that of agar, which can save time. At the same time, the gellan gum is very stable at a high temperature, which is suitable for the cultivation of thermophile microorganisms. In the cultivation test with some plant tissues, it was found that the culture media using gellan gum can speed up the growth of plant tissues. In addition, the amount of gellan gum as used is only ⅓ of the amount of agar. The use of gellan gum can fight against the contamination of mildews. In the replanting process, the cleaning procedure for gellan gum is easy. Further, the use of gellan gum can conduce to the easy observation of the growth of roots and the growth of tissues because the gel formed from gellan gum has a superior transmission of light. Therefore, the low acyl gellan gum also has a broad application promise in the tissue culture medium.
At present, although the food-level low acyl gellan gum produced by industry can be used in the products for tissue culture, the food-level gellan gum produced domestically has a lot of problems in the application for preparing culture medium, if compared with the low acyl gellan gum under the trade name GELZAN™ CM type, provided by CP KELCO Inc., USA and the low acyl gellan gum under the trade name PHYTAGEL Model, provided by SIGMA Corporation. The details are listed as follows:
1. The food-level low acyl gellan gum has a poor solubility and a poor dispensability, is easy to associate, and has a hydration temperature of above 80° C. But, the gellan gum for culture medium produced by oversea companies has a good dispensability and a hydration temperature of about 60° C.
2. The general food-level gellan gum needs to be strictly treated with a specific procedure during the usage process, wherein the specific procedure has the following order of firstly dissolving the colloid and then adding the metal ions into the solution. If the gellan gum powder and the metal salts were firstly mixed, and the mixture of gellan gum power and metal salts were then dissolved, it would lead to a difficulty in dissolving process and would produce a lot of milk-white flocculent. But, the gellan gum provided by oversea companies does not have this problem.
3. The general food-level gellan gum has the gel strength of above 900 g/cm2. But, the gellan gum for culture medium provided by the oversea companies has the gel strength of 400∞600 g/cm2. At the same time, the general food-level gellan gum has a light transmission of about 85% while the gellan gum product for culture medium provided by the oversea companies has a light transmission of above 90%.